1. Field of the Invention
The present invention relates to the production of nanoporous silica dielectric films and to semiconductor devices and integrated circuits comprising these improved films. The nanoporous films of the invention are prepared using silicon containing pre-polymers and are prepared by a process that allows crosslinking at lowered gel temperatures by means of a metal-ion-free onium or nucleophile catalyst.
2. Description of the Related Art
As feature sizes in integrated circuits are reduced to below 0.15 μm and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials used in the manufacture of integrated circuits.
One type of material with a low dielectric constant is nanoporous silica films prepared from silicon containing pre-polymers by a spin-on sol-gel technique. Air has a dielectric constant of 1, and when air is introduced into a suitable silica material having a nanometer-scale pore structure, such films can be prepared with relatively low dielectric constants (“k”). Nanoporous silica materials are attractive because similar precursors, including organic-substituted silanes, such as tetraacetoxysilane (TAS)/methyltriacetoxysilane (MTAS)-derived silicon polymers are used as the base matrix and are used for the currently employed spin-on-glasses (“S.O.G.”) and chemical vapor deposition (“CVD”) of silica SiO2. Such materials have demonstrated high mechanical strength as indicated by modulus and stud pull data. Mechanical properties can be optimized by controlling the pore size distribution of the porous film. Nanoporous silica materials are attractive because it is possible to control the pore size, and hence the density, mechanical strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous films offer other advantages including thermal stability to 900° C.; substantially small pore size, i.e., at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit; preparation from materials such as silica and tetraethoxysilane (TEOS) that are widely used in semiconductors; the ability to “tune” the dielectric constant of nanoporous silica over a wide range; and deposition of a nanoporous film can be achieved using tools similar to those employed for conventional S.O.G. processing.
Thus, high porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. An additional advantage, is that additional compositions and processes may be employed to produce nanoporous films while varying the relative density of the material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous films that controls the dielectric constant of the material, and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the degree of porosity decreases, and vice versa. This suggests that the density range of nanoporous films must be optimally balanced between the desired range of low dielectric constant and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate suitable for the purpose. Usually, a precursor in the form of, e.g., a spin-on-glass composition is applied to a substrate, and then polymerized in such a way as to form a dielectric film comprising nanometer-scale voids.
When forming such nanoporous films, e.g., by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and water to cause polymerization/gelation (“aging”) during an initial heating step. In order to achieve maximum strength through pore size selection, a low molecular weight porogen is used.
U.S. Pat. No. 5,895,263 describes forming a nanoporous silica dielectric film on a substrate, e.g., a wafer, by applying a composition comprising decomposable polymer and organic polysilica i.e., including condensed or polymerized silicon polymer, heating the composition to further condense the polysilica, and decomposing the decomposable polymer to form a porous dielectric layer. This process, like many of the previously employed methods of forming nanoporous films on semiconductors, has the disadvantage of requiring heating for both the aging or condensing process, and for the removal of a polymer to form the nanoporous film. Furthermore, there is a disadvantage that organic polysilica, contained in a precursor solution, tends to increase in molecular weight after the solution is prepared; consequently, the viscosity of such precursor solutions increases during storage, and the thickness of films made from stored solutions will increase as the age of the solution increases. The instability of organic polysilica thus requires short shelf life, cold storage, and fine tuning of the coating parameters to achieve consistent film properties in a microelectronics/integrated circuit manufacturing process.
Formation of a stable porous structure relies on the condition that the porogen removal temperature is higher than the crosslinking temperature (or the gel temperature) of the matrix material. It was found that a stable nanoporous structure of less than 10 nm pore size cannot be produced when the concentration of the alkali cation such as sodium is below 200-300 ppb level in the spin-on solution. However, stringent requirement for low metal concentration must be met for IC applications. The general practice is to have metal concentration below 50 ppb in the spin-on solution. Therefore, there is a need to develop a low metal nanoporous silica film that can consistently give dielectric constant less than 2.5 and pore size less than about 10 nm in diameter. It has now been found that by the use of onium ions or nucleophiles the formation of a porous silica network at lower temperature in a low metal spin-on formulation can be facilitated. The effect of the onium ions or nucleophiles is to lower the gel temperature so that the rigid network is set in before the removal of the porogen, thus producing a nanoporous film without requiring the presence of an undesirable alkali ion.